The '101 greatest questions of all time' have been answered in a BBC magazine. Here's we present 21-40.

Could I be allergic to water?

-> No. Allergies to a substance result when immune system antibodies attack it. But no antibodies exist that bind to water.

How is bullet-proof glass made?-> It might be better to call it ‘bullet-resistant’ glass, because it resists penetration, although a series of bullets fired at the same place will eventually break through. It is usually made by sandwiching a layer of deformable polycarbonate plastic between glass layers. The bullet’s impact shatters the outer layer of glass, spreading the energy over a wide area so the plastic layer ‘gives’, but does not rupture.

How do seeds know which way to grow?-> All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots – and the seed while it is underground – rely on gravity for orientation.

Why are sumo-wrestlers so fat?-> It’s to do with Newton’s second law of motion, which can be written as acceleration = force/mass. The heavier you are, the more force an opponent has to exert to get you moving and push you out of the ring, or to lift and throw you. The heaviest wrestler weighed 267kg, which no weightlifter has ever lifted.

How does Blu-tack stick, but not feel sticky?-> Blu-tack uses a so-called electrostatic interaction, forming a chemical bond between itself and whatever it’s sticking to. It’s made up of very small molecules and, over time, seeps into microscopic indents in the surface of materials. The electrostatic interaction gradually gives way to that molecular ‘seeping’. Hence Blu-tack doesn’t feel sticky initially, but gets stickier if you handle it long enough.

Why do old men go bald, but get hairy nostrils?-> Male pattern baldness is caused by the hormone DHT or dihydro-testosterone. This has many other effects on the body and it is thought to inhibit hair loss in the nostrils so that each shaft has a longer growing phase. But the exact mechanism is still not well understood.

What is the Universe made of?-> This is one of the biggest mysteries facing 21st-century science. Recent studies of the heat left over from the Big Bang have revealed that conventional matter made from atoms accounts for just a few per cent of the matter in the Universe. Most of it is in the form of so-called cold dark matter (CDM), whose enigmatic name reflects the awkward fact that astronomers don’t know what it is.

Why does breathing pure oxygen kill you?-> Our blood has evolved to capture the oxygen we breathe in and bind it safely to the transport molecule called haemoglobin. If you breathe air with a much higher than normal O2 concentration, the oxygen in the lungs overwhelms the blood’s ability to carry it away. The result is that free oxygen binds to the surface proteins of the lungs, interferes with the operation of the central nervous system and also attacks the retina.

Will my eyes pop out if I don’t close them when I sneeze?-> No, for a whole variety of reasons, including the fact that the air spaces in the nose and throat involved in sneezing aren’t directly connected to anything behind the eye. That means they can’t create the pressure to push your eyeballs forward.

If humans died out, would we evolve again from apes?-> Maybe, if we were wiped out by a species-specific plague, it’s possible that another ape species might go on to evolve greater intelligence and develop a fully bipedal gait.

Do any wasps make honey?-> No. Adult wasps drink nectar from flowers but don’t turn it into honey. They feed their young by laying eggs inside prey.

What makes me feel hungry?-> The vagus nerve sends information about the fullness of the stomach to the hypothalamus in the brain. Hunger is also controlled by hormones, including insulin, leptin, ghrelin and cholecystokinin. Levels of these change according to the levels of various nutrients in the blood and digestive tract.

How does Viagra work?-> Viagra is the brand name for sildenafil citrate, a drug that was originally studied for use in patients with high blood pressure or heart disease. It works by blocking the action of an enzyme called PDE5. The role of this enzyme is to break down another enzyme called cGMP, which relaxes the smooth muscle of the corpus cavernosa (a mass of erectile tissue in the penis), which allows them to become filled with blood and therefore create an erection. When the sildenafil citrate blocks the PDE5, the cGMP levels continue to build so long as the man remains sexually aroused and thus the corpus cavernosa are inflated with blood to a much higher pressure than they would otherwise be. Smooth muscle relaxants have been used to create spontaneous erection as early as the 1980s. But what makes Viagra special is that it acts only when the man is aroused, and only on the penis. This is because the PDE5 enzyme is specific to the penis.

Why are flies attracted to bad smells?-> The smells that are a warning of putrefaction to us are attractive to flies for just the same reason. When organic matter (animal or vegetable) starts to rot, it becomes soft and moist, and provides ideal breeding conditions for fly larvae. Female flies are attracted to rotting material, where they lay their eggs in batches (around 50 to 100 in the case of the house fly). The eggs can hatch within 12 hours, and the larvae burrow into the soft material and feed for several days before turning into pupae, which then produce adult flies. It can all take as little as 10 days from egg to adult. LF

After bumping my head, why do I ‘see stars’?-> The stars you see are actually neurons in your visual cortex firing spontaneously. This occurs when their oxygenation level changes abruptly either because you have stood up too quickly or because your brain has been suddenly accelerated by a sharp blow, sloshing blood into or out of the capillaries. The neurons closest to capillaries are affected first and, if it happens fast enough, they fire well before the surrounding neurons. This results in isolated signals that your brain interprets as lights.

Why don’t birds get electrocuted while perching on power lines?-> It’s not voltages per se that kill, but voltage differences, which are needed for electricity to flow. If a bird lands on a single power line at, say 35,000 volts, the lack of a voltage difference keeps it safe. If it extends its wings and touches another power line at a different voltage, though, it will be electrocuted. That’s why electricity companies put plenty of space between cables.

Why does sunshine make me happy?-> There’s a fine balancing act in the brain and circulatory system that regulates our moods, boosting the feel-good factor or sometimes causing Seasonal Affective Disorder (SAD) in winter. The hormone melatonin, produced by the pineal gland in the brain, regulates our sleep/wake cycle, but excessive levels can cause depression. The action of light on our face suppresses melatonin synthesis, therefore boosting your overall mood. Evolutionary principles could apply too. There are plenty of nocturnal primates and presumably they aren’t always depressed. It seems likely that we have evolved to prefer light to darkness because humans are active during the day.

Is eating Red grapes as good for you as drinking red wine?-> No. The benefits of red wine are largely down to ‘flavonoids’, which are twice as concentrated in wine as in grape juice.